Plastic composites are widely used for aerospace applications where high strength, low weight, good heat resistance and ablative properties are required. Examples of these applications include rocket nozzle liners, rocket motor case liners, and reentry vehicle heat shields, nose tips and leading edges. The plastic composites generally consist of a fibrous reinforcement such as graphite or quartz fabric and a resin matrix such as phenolic or polyimide. A problem associated with use of these composites is that the upper temperature at which they can be used is generally limited by the heat resistance of the resin. For example, the compression strength of a composite of threedimensionally woven graphite fabric impregnated with phenolic resin may be 40,000 psi at room temperature but only 2,000 psi at 700.degree. F. This loss of strength is primarily due to degradation of the phenolic resin, not of the graphite fabric. Decrease in strength of graphite fabric alone at 700.degree. F is insignificant. Another composite property that can be limited by the resin is ablation resistance. This limitation is frequently associated with the limited heat resistance of the resin.
Several approaches for alleviating or circumventing this problem have been attempted in the past. One approach was to develop resins with increased heat resistance. Although many new resins such as polybenzimidazoles, polyquinoxalines and pyrrones have been made available, they are generally very difficult to process, and their heat resistance is still considerably less than that of many reinforcement materials. Another approach was to increase the reinforcement content of composites thereby decreasing the resin content by adding particulate or fibrous fillers such as carbon or silica during processing. This method has had only limited success because the fillers are not mechanically locked or adhered to the basic reinforcement and therefore contribute little to the strength or ablative properties of the composite. A third approach was the development of composites in which both constituents, i.e., the reinforcement and the matrix, were high temperature ceramic materials. Examples are carbon-carbon and quartz-silica composites. These materials have excellent heat resistance and ablative properties but are generally too brittle or too thermally conductive for many of the applications mentioned above.
Carbon-carbon and carbon-phenolic materials are examples of fiber reinforced composites that have been highly developed by the aerospace community for use as thermal protection systems and have generally been shown to be the lightest in weight for high ballistic coefficient reentry vehicles. Important attributes of carbon-carbon composites include low ablative recession, smooth ablative surfaces, and high strength at elevated temperatures. However, application of these attributes presents severe design problems because carbon-carbon materials have (1) very high thermal conductivity which results in high internal temperatures, (2) low tensile strain to failure which can result in thermal stress-cracking problems from reentry heating, and (3) low structural response resistance to impulsive loads. On the other hand, carbon-phenolic materials have significantly lower thermal conductivity, moderate tensile strain to failure, and low thermal expansion. These characteristics simplify design by yielding low internal temperatures and minimizing the thermal stress problem; but this is done at the expense of increased ablation and loss of strength above 600.degree. F.
U.S. Pat. No. 3,778,336 to Adams discloses a method for preparing light-weight carbonized structures by coating, for example, a polyurethane foam with a resin layer, subjecting the resulting coated structure to pyrolysis, and then coating the pyrolyzed structure with an oxidation-resistant organic resin. This patent is directed to a surface coating method and does not involve fabrication of a fiber reinforced material according to the present invention concept.
U.S. Pat. No. 3,796,616 to Northway discloses a method for production of fibrous graphite structures by incorporating fugitive or degradable fibrous material between plies of resin impregnated cloth, curing the resin and thereafter disintegrating the fibrous material, leaving a porous substrate which is subsequently infiltrated with pyrolytic carbon. This procedure does not produce a mixed carbon and resin matrix, that is a bimatrix material, in accordance with the present invention.
This invention relates to the production of plastic composites formed of fiber and resin, having improved strength at high temperatures, and improved heat resistance, while maintaining low weight, and is particularly concerned with the production of composites of the above type composed of a mixture of pyrolyzed and non-pyrolyzed resin, e.g., phenolic, matrices reinforced with carbon or graphite yarns, preferably in the form of a woven fabric, and forming a bimatrix composite; and the procedure for producing such composites.